1. Field of the Invention
The present invention relates to a process for preparing pyromellitic dianhydride (PMDA) by heterogeneously catalyzed oxidation in the gas phase by means of a gas containing molecular oxygen and to catalysts to be used in this process.
2. The Prior Art
PMDA has, up to now, been obtained on a large scale mainly by liquid-phase oxidation of 2,4,5-trimethylbenzaldehyde with atmospheric oxygen, in a process analogous to the process described in DE-A 1,943,510 (GB-A 1,282,775) for preparing terephthalic acid from p-toluylaldehyde, in which the pyromellitic acid thus obtained is dehydrated to PMDA. The 2,4,5-trimethylbenzaldehyde is prepared by carbonylation of 1,2,4-trimethylbenzene (pseudo-cumene) (DE-A 2,422,197=GB-A 1,422,308). The use of acetic acid as a solvent and heavy-metal salts in combination with a bromide source (Chem-Systems Report: PERP 1987-T-4, 16-40) as catalysts necessitate, in this process, the use of high performance and therefore very expensive alloys (Hastelloy C) for the reactor. Besides the batchwise operation, a further disadvantage of this process is that the pyromellitic acid obtained by liquid-phase oxidation must be dehydrated to PMDA in a very energy-intensive step (&gt;200.degree. C.).
A further process for preparing PMDA employing the principle of liquid-phase oxidation is the Amoco process (U.S. Pat. No. 4,719,311). Using a similar catalyst (Co-Mn-Br), 1,2,4,5-tetramethylbenzene (durene) is oxidized with atmospheric oxygen to pyromellitic acid, which likewise still has to be dehydrated to PMDA. In addition to the disadvantages described for the above-mentioned process, this process has the further disadvantage that durene is above five times more expensive than pseudo-cumene.
A third PMDA process operates in the gas phase. Analogous to the oxidation of o-xylene to phthalic anhydride, durene is oxidized directly to PMDA over a heterogeneous catalyst in a multiple-tube reactor. U.S. Pat. No. 4,665,200 discloses V.sub.2 O.sub.5, TiO.sub.2, P.sub.2 O.sub.5, Nb.sub.2 O.sub.5, Sb.sub.2 O.sub.3, K.sub.2 O and Cs.sub.2 O as catalyst components. Particular advantages of the gas-phase oxidation are continuous operation and the easy handling of the catalysts, as a result of which the use of expensive materials in plant construction can be dispensed with. In principle, it is possible to inexpensively retrofit existing plants for PMDA production. The energy-intensive dehydration of the liquid-phase process is eliminated, since the anhydride is desublimated directly from the reaction gas. A PMDA purity of 99% is achieved by means of suitable desublimation processes (DE-A 3,730,747=U.S. Pat. No. 4,867,763).
A further way of obtaining PMDA from the reaction gas is gas scrubbing with an anhydrous solvent, a technology which is state of the art in, for example, the preparation of maleic anhydride (SRI International, PEP Report 46C, 1989). Further examples of the preparation of PMDA via gas-phase oxidation in the presence of vanadium- or titanium-containing catalysts are described in EP-A 405,508 and EP-A 330,195. A disadvantage of the gas-phase oxidation when compared with the previously known processes is the lower selectivity in comparison with liquid-phase oxidation.
For the gas-phase oxidation, starting materials which have been described are, in addition to 1,2,4,5-tetraalkylated benzenes, functionally substituted benzene derivatives which are prepared from trisubstituted benzenes, for example pseudo-cumene. Functional groups described are chloromethyl and alkoxymethyl (AT-PS 169 330). For ecological reasons, chlorine-containing aromatics are questionable, especially at such high reaction temperatures as occur in the gas-phase oxidation. Alkoxymethylbenzenes are likewise produced via a chloromethylation and are to be avoided for the same reason.
In summary, the most serious disadvantages of the processes known from the prior art are, for the liquid-phase oxidation processes, the expensive reactor materials because of the corrosive catalysts, the long down-times as a result of batchwise operation, the energy-intensive dehydration of the acid to the anhydride and, for the gas-phase oxidation processes, their expensive raw material base and low selectivity.